The invention relates to a superconducting composite conductor with stabilizing material and several superconducting conductor strands which consist of an intermetallic compound developed in a diffusion reaction of at least two elements and are embedded in a normally-conducting matrix which is surrounded by an aluminum jacket and contains copper as the alloying component of at least one of the elements of the compound, where at least one alloying element of the matrix is separated by a barrier from the stabilizing material. Composite superconducting conductors are known, for instance, from "IEEE Transactions on Magnetics", Vol. MAG-19 No. 3, May 1983, pages 672 to 675. The invention further relates to a method for manufacturing such a composite superconducting conductor. Superconductors suited for technical applications always contain in addition to a superconducting material component a normally-conducting component of high conductivity for stabilizing the superconducting state. If a local temperature rise occurs in such a conductor, for instance, as a result of jumps in the flux, the current-carrying capacity of the superconductor is reduced in the region under discussion or disappears altogether indeed, but the current can be carried for a brief moment by the normally-conducting component, so that the superconductor has the opportunity to recover. This process is therefore also designated as a stabilization means against transient disturbances. With the stabilizing component an excessive temperature rise of a coil wound with such a superconductor and its possible destruction can therefore be prevented.
For stabilizing superconductors, high-conductivity copper (Cu) is generally used which, with a so-called external stabilization, surrounds the superconductor in the form of a jacket, or, in the case of a so-called internal stabilization, is arranged as at least one core in the center of the conductor. Besides copper, aluminum (Al) is gaining increasing importance as a normally-conducting component for the stabilization of superconductors.
The conductivity of aluminum as compared with that of copper is substantially higher at 4.2.degree. K. in a space free of magnetic field and also decreases considerably less in a magnetic field with increasing flux density than the conductivity of Cu. Thus, at 4,2.degree. K. resistivities of 2.times.10.sup.-9 ohm-cm and 6.times.10.sup.-10 ohm-cm, respectively, are measured on samples of soft-annealed Cu 99.997% and soft-annealed Al 99.995%. At 4.2.degree. K. and with a magnetic flux density of 12 tesla, the resistivity of copper samples is about 26/times higher that of aluminum samples, but only 6/times higher than in the absence of a magnetic field.
Such composite superconducting conductors, the current-carrying component of which is a brittle, nondeformable intermetallic compound, can be produced particularly by a special method, the so-called bronze process. In the case of the intermetallic compound Nb.sub.3 Sn, prefabricated composite wires with the final dimensions of the desired conductor, consisting of niobium wires or filaments in a matrix of copper-tin bronze are annealed at a temperature of about 700.degree. C. for a predetermined time. Tin dissolved in copper, where in general in the phase, reacts in a solid reaction with the niobium, forming the desired intermetallic compound Nb.sub.3 Sn. If such bronze conductors are stabilized, as customary, with copper, the bronze of the matrix and the copper of the stabilizing cross section must be separated by a special intermediate layer, for instance, of niobium or in particular, of tantatum. Such metals then act as a diffusion barrier (see, for instance, DE-OS No. 23 39 525) and prevent, during the annealing reaction to develop the desired super-conducting compound, the penetration of tin into the stabilizing copper and thereby resulting in a reduction of the latter's electric and thermal conductivity. Admixtures as small as 0.05% by weight of tin reduce the residual resistance ratio of copper from about 1,000 to a value of about 20. With electrically and therefore also thermally poorly conducting copper, however, a sufficient stabilizing effect on the superconducting state is not attainable.
Next to copper as the stabilizing material of such composite conductors, stabilization with aluminum is also known. For this purpose the aluminum can be soldered, after the reaction, on a twisted conductor by means of lead-tin solder (see, "IEEE Transactions on Magnetics", Vol. MAG-17, No. 5, September 1981, pages 2047 to 2050).
Composite conductors of bronze and niobium filaments with a central core of aluminum can also be fabricated, where this core is surrounded by a diffusion barrier of niobium (see, for instance, "IEEE Transactions on Magnetics", Vol. MAG-21, No. 2, March 1985, pages 157 to 160). With this conductor type, however, the annealing reaction must be carried out below the melting point of aluminum.
Another method for stabilizing a superconductor composite conductor with aluminum is found in the publication mentioned at the outset "IEEE Trans. Magn." Vol. MAG-19. According to this method, a preliminary conductor product is first fabricated by embedding a predetermined number of wires of the one element of the compound, especially niobium wires, in a matrix which contains the remaining element or elements of the compound in the form of an alloy and thus consists of a tin-bronze, for instance. In addition, this structure contains, particularly on its outside, also copper as the stabilizing material which is separated from the bronze material in a manner known per se by diffusion-retarding or -inhibitory barriers. After this structure is worked to the desired final form, the desired superconducting compound such as Nb.sub.3 Sn is developed in a diffusion reaction by means of a heat treatment. The superconducting core is finally provided with an aluminum jacket by extrusion; a good metallurgical bond between the copper and the aluminum can be achieved in this manner.
With the superconducting composite conductors produced by this known method, the diffusion barriers thus require part of the normal-conducting cross section, so that in this conductor type the effective current density J.sub.ov is limited accordingly. Furthermore the cross section cannot be reduced arbitrarily since the niobium or tantalum tubes used as barrier material have a tendency to burst if their wall thickness is too small, as would occur with the usual high degrees of deformation of the raw conductors.